Pressedboard, frequently referred to also as fiberboard or particleboard, is generally formed by hot pressing a mat or web of a comminuted material. This material may contain a thermally activatable intrinsic binder or which may be admixed with a thermally activatable binder to produce a relatively rigid product which can range from extremely high density structural board to comparatively low density porous board which can be used, for example, for insulation.
The art has recognized a wide variety of materials for the production of pressedboard. Typical materials are sawdust and wood chips for the production of particleboard, wood fibers or other cellulosic fibers in the production of fiberboard.
The hot pressed product may be used directly or may form a core between layers or have layers applied to one surface thereof in order to face the board and/or provide patterns thereof, or to improve the surface quality of the board.
Frequently, during the fabricating process, the board will be embossed and various contours will be imparted to the finished product.
The synthetic resin binders which can be used are generally those which are considered thermosetting, e.g. phenol-formaldehyde, melamine, urea and resorcinal resins.
Much of the technology in the field involves the use of conveyors on which the layers of comminuted material are formed, means for subdividing such layers into mats, and platen presses operated with or without mat-carrying trays for the pressing and handling of mats and board. However, the production of a continuous strip of the comminuted material has received considerable interest because of its utility in the production of a pressedboard.
It is known, for example, to form such a strip by feeding the comminuted materials upon a conveyor surface between a pair of platens.
In a more or less continuous process, the layers can be received between a pair of continuous steel belts, namely an upper pressing belt and a lower pressing belt, circulated between the platens and therefore receiving the layer between them as they meet to transport the layer between the platens.
When pressboard is to be formed by thermally activating the binder between these platens, the platens may be heated. Of course, it is also possible to use this system as a preliminary press and to thermally activate the binder downstream of the press to form the board.
In either case, the surfaces engaged by the layers are those of the steel bands and the steel bands are pressed against the rigid surfaces of the platen by rollers, rods or bars, which are circulated through the gaps between the steel bands and the platens.
The circulation of these pressing bars, which are hereinafter referred to as roller bars, can be effected by a relatively complex mechanism of the type described, for example, in German application 22 15 618.
The circulating means of this arrangement include guide rails which define the circulating path for the bars and which engage the ends thereof. Within the gap between the pressing band and the platen, at least, the movement of the roller bars is accomplished with one bar pressing against the next. This is, in a sense, a passive roller transport system since the impetus for movement of the rollers is drawn from the displacement of the steel belt. A plunger arrangement or the like can be provided to introduce the rollers into the gap between the steel belt and the platen.
This system has some highly significant disadvantages. Firstly, canting of the roller bars is unavoidable, there being a tendency from time to time for one end of a bar to lead or trail the opposite end thereof. This can result in jamming and interruption of the movement of the rollers through the gap. In practice, such jamming damages the rollers as well as the band and distorts the product. The downtime of the machine necessary to clear the jam is also associated with the need to dispose of a defective portion of the product and, indeed, to replace damaged parts.
Another disadvantage is the comparatively high friction which is generated as the rollers rub upon one another.
It is also a disadvantage that, upon the development of play in the guide system, there is a tendency for upstream rollers to jump over rollers which are further downstream.
It might be thought that the problem could be resolved by coupling the rollers or bars together in a chain, i.e. by providing common tractive means for drawing the rollers through the gap. This would provide an active transport of the rollers. Experience with such systems, however, has shown that this introduces additional stress factors in the pressing zone which increase the tendency to friction, corrosion, slip and the like, all of which render the system impractical.